Effect of Pump Type on the health of various CMP slurries


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Effect of Pump Type on the health of various CMP slurries

  1. 1. FT33- 06_5 CRITICAL COMPONENTS & SUB SYSTEMS Effect of pump type on the health of various CMP slurries Mark R. Litchy, Donald C. Grant, CT Associates, Inc., USA, & Reto Schoeb, Levitronix GmbH, Switzerland Source: Mykrolis, MIT LL ABSTRACT Delivery systems used to supply slurry to CMP planarization tools can damage slurry. In this experiment, four slurries were circulated in a simulated slurry delivery loop at a fixed flow rate and pressure using a variety of pumps (bellows, diaphragm, and magnetically levitated centrifugal) to determine the effect of circulation on the slurry health. During each test, a number of slurry health parameters were monitored including the size distribution of the particles in the slurry. Most slurry health parameters were unaffected during the tests. However, significant changes in the large particle tail (particles ≥ 0.5µm) of the slurry particle size distributions Figure 1. Test system schematic. (PSD) were observed. Both the pump Experimental procedure accepted as low shear devices due to and slurry type played important their relatively low speeds of operation. roles in the magnitude of the change. A schematic of the test system used Centrifugal pumps have not been widely In some slurries, large increases in in this evaluation is shown in Figure used for delivery of agglomeration- the large particle concentrations 1. Each pump was used to circulate sensitive slurries since they are perceived were observed during circulation 12 liters of slurry at a flow rate of as high shear devices, due to their with diaphragm and bellows pumps, approximately 26-30 lpm (7-8 gpm) and relatively high speeds of operation. while in other slurries increases were outlet pressure of approximately 30 psig Previous studies have shown significant not observed. With the magnetically (2.1 bars). Settling of the slurry in the increases in large particle concentrations levitated centrifugal pumps, minimal tank was minimized by drawing from when certain types of pumps (diaphragm changes were observed, regardless of the bottom of a conical bottom tank and bellows) were used to circulate the type of slurry tested. and by turning the volume of slurry in slurry [2-5]. However, these tests were the tank over in less than 30 seconds. performed using a slurry known to The return line to the slurry tank was be sensitive to particle agglomeration. submerged below the liquid level of Introduction In practice, there are many CMP the slurry to avoid entraining gas into planarization applications incorporating Delivery systems are often used to the slurry. The return line was also many different types of slurries. supply slurry during wafer planarization. positioned to minimize the formation While it is not feasible to evaluate all These systems pressurize and circulate of a large vortex in the tank, which can available slurries with all types of pumps, the slurry to deliver it to the tools entrain gas into the slurry. No valves this set of experiments was undertaken and keep the particles in suspension. were used to generate backpressure at to determine the effect of pump type Pressur ization and circulation are the outlet of the pump. Instead, a long on the health of a small number, but accomplished by var ious means length of 1⁄2quot; PFA tubing was used to wide variety, of slurry types. The slurries including pumping and pressure- gradually reduce the pressure at the selected for this experiment include vacuum technology. Typically, the slurry pump outlet to ambient pressure at the a fumed silica slurry, a colloidal silica passes through the equipment providing end of the return line to the tank. slurry, an alumina slurry, and a ceria the motive force approximately The air pressure supplied to the slurry. The pumps in this experiment 100 times before it is used to polish diaphragm and bellows pumps was included a diaphragm pump, a bellows wafers, i.e. the slurry is ‘turned-over’ adjusted to achieve the desired flow pump, and two magnetically levitated approximately 100 times [1]. Some rate and outlet pressure. Meanwhile, the centrifugal pumps. The sizes of the CMP slurries are susceptible to damage speed of the small and large centrifugal pumps were chosen such that each was caused by mechanical handling. For pumps was varied to achieve the desired capable of delivering slurry at a flow example, particle agglomerates may flow rate and pressure. In each test, the rate of 30 lpm at an outlet pressure of 30 form that can limit the life of filters or slurry was circulated until approximately psig. Manufacturer-recommended pulse reduce yield by causing wafer defects. 1,000 tank turnovers were achieved. dampeners were installed downstream Diaphragm and bellows pumps The test system was constructed of PFA, have been commonly used for bulk of both the bellows and diaphragm except for the conical bottom tank that CMP slurry delivery. These pumps are pumps to minimize pulsation. was constructed of polyethylene. The 2 SEMICONDUC T O R F A B T E C H – 3 3 RD E D I T I O N W W W .FABTECH .ORG
  2. 2. FT33- 06_5 CRITICAL COMPONENTS & SUB SYSTEMS Barbara, CA) that determines particle size by dynamic light scattering. The size distribution of the large particle tail was measured using a Particle Sizing Systems AccuSizer 780 sensor. The AccuSizer 780 sensor uses a combination of light scattering and light extinction to measure the size distribution of particles ≥ 0.56µm. The size measurements were performed by injecting the slurry sample into a flowing stream of filtered deionized water using the test system shown in Figure 2. The dilution ratio was varied by adjusting the slurry injection rate. The slurry types required different dilution ratios, which varied from approximately 800:1 to Figure 2. Slurry dilution system schematic. 32,000:1. Prior to starting the pump tests, each slurry was thoroughly analyzed to slurry used in each test was taken from dehydration can both result in particle determine the proper dilution ratio for the same lot of slurry. agglomeration in the slurry. A chiller and accurate measurement of the large particle The tank holding the slurry was stainless steel coil were used to maintain tail of the slurry PSD. Each slurry contains blanketed with nitrogen to prevent the slurry at 22 ± 2°C during the test. a very high concentration of the ‘working’ absorption of carbon dioxide from the Samples were drawn from the system particles, which are responsible for the air, which can change the pH of the at selected times for analysis. The PSD mechanical portion of the polishing. Care slurry. The nitrogen was humidified to was measured using two techniques. The must be taken to ensure that each slurry prevent dehydration of the slurry. The size of the ‘working’ particles (typically ~ is diluted sufficiently such that scattering relative humidity in the tank was > 90% 0.05-0.5µm) was measured using Particle from particles < 0.56µm does not interfere throughout the test. Shifts in the pH and Sizing Systems’ NICOMP 380ZLS (Santa with the particle size analysis. Figure 3. Cumulative PSDs measured in the fumed silica slurry. 3 S E M I C O N D U C T O R F A B T E C H – 3 3 RD E D I T I ON
  3. 3. FT33- 06_5 CRITICAL COMPONENTS & SUB SYSTEMS Figure 4. Cumulative PSDs measured in the alumina slurry. Figure 5. Cumulative PSDs measured in the colloidal silica slurry. 4 SEMICONDUC T O R F A B T E C H – 3 3 RD E D I T I O N W W W .FABTECH .ORG
  4. 4. FT33- 06_5 CRITICAL COMPONENTS & SUB SYSTEMS Figure 6. Cumulative PSDs measured in the ceria slurry. The working PSD and zeta potential measured with each slurry. The four concentration of particles ≥ 0.56µm graphs in each figure present the results increased during the diaphragm and measurements were both made using from the four pumps tested. The initial bellows pump tests, but to a lesser the NICOMP 380ZLS. The samples PSD, measured prior to the start of each extent than during tests with the fumed were diluted approximately 40:1 into test, is presented in each graph as well silica and alumina slurries. deionized water and analyzed at 23°C. as the PSDs after selected numbers of As with the previous slurries, no Each PSD measurement was made over turnovers. changes were observed during the a period of 10 minutes, while each zeta The fumed silica slurry, which has centrifugal pump tests in the ceria slurry potential measurement was made over been known to be sensitive to particle (Figure 6), while significant increases 2 minutes. The PSD and zeta potential agglomeration [2-5], exhibited large in the concentration of particles larger measurements of each sample were increases in the concentration of large than about 2µm in size were observed performed in triplicate and quintuplicate, particles when subjected to multiple during the diaphragm and bellows respectively. The size measurement data passes through the diaphragm or bellows pump tests. were analyzed using the instrument’s pumps (Figure 3). The concentration To better visualize the changes in gaussian distribution assumption. increases occurred over a wide particle the PSDs during the tests, the ratios of Other slur r y health parameters size range from ≥ 0.56µm to more than the particle concentrations at each test measured included zeta potential, total 10µm. However, when this slurry was point to the corresponding particle percent solids, specific gravity, and pH. subjected to many passes through the concentration at the start of each Measurements of each sample were magnetically levitated centrifugal pumps, test were plotted. An example of the performed in triplicate. little change in the concentration of results is presented in Figure 7 for the Results and discussion large particles was observed. alumina slurry tests. The four graphs in Similar to the fumed silica slurry, Figure 7 show concentration ratios for The results presented will focus on little change was observed dur ing selected size channels as a function of the measurements of the slurry large the centr ifugal pump tests in the tank turnovers during each pump test. particle tail since substantial changes alumina slurry as shown in Figure 4. The results for the other three slurry were observed in the tail during these However, large increases in the particle tests are included in reference [6]. In this experiments. Results for the other concentration, particularly for particles slurry, significant changes in the large parameters were not included due to > 1µm, were observed dur ing the particle tail, particularly for particles ≥ the fact that none of those parameters diaphragm and bellows pump tests. 1.5µm in size, were observed during the changed significantly during the 1,000- For the colloidal silica slurry (Figure diaphragm and bellows pump tests in less turnover test. 5), little change was observed during than 10 turnovers. The concentration Figures 3-6 present the cumulative the centrifugal pump tests, while the of particles ≥ 1.5µm tended to increase PSDs of the slurry large particle tail 5 S E M I C O N D U C T O R F A B T E C H – 3 3 RD E D I T I ON
  5. 5. FT33- 06_5 CRITICAL COMPONENTS & SUB SYSTEMS Figure 7. Concentrations relative to initial concentration during the alumina slurry tests. Figure 8. Concentration increases measured during all tests after 100 turnovers. 6 SEMICONDUC T O R F A B T E C H – 3 3 RD E D I T I O N W W W .FABTECH .ORG
  6. 6. FT33- 06_5 CRITICAL COMPONENTS & SUB SYSTEMS Figure 9. Concentration increases measured during all tests after 1,000 turnovers. linearly with increasing turnovers. After TABLE I: SUMMARY OF CONCENTRATION CHANGES IN SUBMICRON 1,000 turnovers, the concentration of AND SUPERMICRON PARTICLE SIZES DURING ALL TESTS particles ≥ 2µm increased approximately 35- and 70- fold during the diaphragm and bellows pump tests, respectively. This equates to about a 3.5% and 7% increase in particle concentrations each time the slurry passes through these pumps, respectively. Meanwhile, concentrations measured during both centrifugal pump tests tended to decrease with increasing turnovers. Figures 8 and 9 present a subset of the data collected in this study. The data are organized by slurry type rather a decrease in the particle concentrations in three of the four slurries tested. In than by pump type to more easily was observed in some of the slurries the fumed and colloidal silica slurries, compare the effects of the pumps on during the centrifugal pump tests. For the concentration increases occurred each of the slurries. For each slurry, example, in the alumina slurry centrifugal over a wide range of particle sizes, from the concentration ratios are presented pump tests, the particle concentrations 0.56µm to larger than 10µm. However, as a function of particle size for each decreased on the order of 30-50% for concentration increases were very large pump after approximately 100 and particle sizes ranging from 0.56µm to during the diaphragm and bellows 1,000 turnovers. These points in time more than 5µm. Ratios were not plotted pump tests in fumed silica slurry, while were chosen since slurry is typically for some of the large particle sizes in only small increases were observed in turned over on the order of 100 times the alumina and ceria centrifugal pump the colloidal silica slurry. Meanwhile, in prior to use, with 1,000 probably being tests since the particle concentrations the alumina and ceria slurries, little or a conservative upper limit in most were so low, but no increases in particle no particle concentration increase was delivery systems. concentrations were apparent. observed for particles < 1.0µm in size, There were no observed significant Meanwhile, the concentrations of but very large concentration increases increases in the particle concentrations particles dramatically increased during were observed with both the diaphragm with either centrifugal pump at any the diaphragm and bellows pump tests and bellows pumps for par ticles particle size in any of the slurries. In fact, 7 S E M I C O N D U C T O R F A B T E C H – 3 3 RD E D I T I ON
  7. 7. FT33- 06_5 CRITICAL COMPONENTS & SUB SYSTEMS ≥ 2.0µm in size. (For the ceria slurry, the four slurries tested (fumed silica, REFERENCES alumina, and ceria) during both the the actual concentration ratios are likely [1] Personnel communication with J. diaphragm and bellows pump tests. For to be even higher than indicated for Kvalheim, BOC Edwards Chemical the fumed silica slurry, the concentration particles ≥ 3µm, since the initial particle Management Division, Chanhassen, increases were much greater than an concentration for these large sizes was MN, March 2003. order of magnitude for both submicron near the background of the test system.) [2] Litchy MR and R Schoeb, ‘Effect and supermicron particle sizes. The particle sizes at which of shear stress and pump method concentration increases occur red Summary on CMP slurry,’ Semiconductor during the diaphragm and bellows International, 27 (12), 87-90 (2004). pump tests were remarkably similar in Previous work has shown that silica- [3] Nicholes K, R Singh, DC Grant, and all slurries. In general, the concentration based CMP slurries are sensitive to MR Litchy, (2001). ‘Measuring particles agglomeration induced by extensive increases observed during the bellows in CMP Slurries,’ Semiconductor slurry handling. This study has shown pump tests were larger than those seen International, 24(8): 201-206. that non-silica-based CMP slurries during the diaphragm pump tests. The [4] Litchy MR and R Schoeb (2005), such as alumina and cer ia are also concentration increases observed with ‘Effect of particle size distribution on sensitive. Both slurry type and pump the bellows pump were typically about filter lifetime in three slurry pump type are key factors influencing the twice the level of increase with the systems,’ Materials Research Society magnitude of agglomeration during diaphragm pump, except during the Symposium Proceedings Vol. 867, slur ry handling. Minimal changes ceria slurry tests in which the increases W2.8.1, (2005). in the large particle tail of the slurry appeared to be even higher. [5] Litchy MR, DC Grant and G Van PSD were observed during tests with Table I shows a summary of the Schooneveld (2006). ‘Effects of fluid magnetically levitated centr ifugal concentration changes relative to the handling components on slurry pumps, while large increases in the initial concentration during each test health,’ Transaction on Electrical and large par ticle tail were obser ved for both submicron (0.56 to 1.0µm) Electronic Materials of the Korean in silica, alumina, and cer ia-based and super micron (≥ 3µm) particle Institute of Electrical and Electronic slurries during circulation tests with sizes. Minimal changes in particle Material Engineers. either diaphragm or bellows pumps. concentrations were observed during [6] Litchy MR, DC Grant and R Schoeb The bellows pump caused more the 1,000 tur nover test for both (2007). ‘Effect of pump type on agglomeration than the diaphragm centr ifugal pumps. For submicron various CMP slurries,’ Proceedings of pump during most of the slurry tests. particles sizes, minimal changes were the 26th Annual Semiconductor Pure No significant changes were observed also observed with the diaphragm Water and Chemicals Conference, in the other slurry health parameters and centrifugal pumps in both the Sunnyvale, CA, pp 119-134. (working PSD, zeta potential, total alumina and ceria slurries. However, percent solids, pH, and specific gravity) concentration increases greater than Reprinted with permission from the for any of the pumps during any of the an order of magnitude were observed SPWCC (Semiconductor Pure Water slurry tests. for supermicron particles in three of and Chemicals Conference). ABOUT THE AUTHORS ENQUIRIES Mark Litchy is a Research Engineer for CT contamination control, particle measurement CT Associates, Inc. 7121 Shady Oak Road, Eden Prairie, MN 55344 Associates, Inc. He has more than 10 years of and control, filtration, permeation and chemical USA experience in particle measurement and control engineering. He has more than 30 years of Tel: +1 (952) 470-0166 in high purity liquid chemicals and gases and experience in analysis and purification of fluids Website: www.ctassociatesinc.com CMP slurry characterization. He is the author of and is the author or co-author of more than 130 more than 20 publications and presentations. He technical papers and presentations. He has a has a M.S. in mechanical engineering from the M.S. in mechanical engineering from the Particle Levitronix GmbH Particle Technology Laboratory at the University of Technology Laboratory at the University of Technoparkstrasse Minnesota and a B.A. in physics from St. John’s Minnesota and a B.S. in chemical engineering from CH-8005 Zurich University. Case Western Reserve University. Switzerland Don Grant is Founder and President of CT Reto Schoeb is the CEO and Founding Member of Website: www.levitronix.com Associates, Inc., which performs contract research Levitronix GmbH, a pioneer in bearingless pump and development and testing services related to technology. 8 SEMICONDUC T O R F A B T E C H – 3 3 RD E D I T I O N W W W .FABTECH .ORG